I often forget how to do this so, as a quick note to my future self, next time I have to do this. Here is how I fill in a “hole” on a surface model. This can be done by clicking two opposing lines or edges.
Since my objective here is to fill the hole so that the properly merge the solid with the surface. After created that blend, I merge the Copy 1 surface feature with the new Boundary Blend
This creates the Merge 1 feature that should now be selected for further surface merges.
Since my objective is to fill the hole, I selected the next gap edges. This filled without any issue.
To merge the new boundary blend with the previous merging of surfaces, Select Merge 1 and Boundary Blend 2, that was just created. Your numbers might vary, of course.
That will produce Merge 2. Now future surface merging should be done with the latest Merge.